London (PTI): The haunting call of the world's biggest animal -- the blue whale -- is the most intense of any living creature. But, according to a new research, these songs are increasingly getting deeper.

Marine biologists have found that the tone of rhythmic pulses and moans of the blue whales has become steadily lower as their population have slowly recovered after nearly being wiped out by whaling.

According to Professor John Hildebrand, who led the team which carried out underwater recordings of the endangered mammals, blue whale calls have decreased to around 15 Htz last year as compared to a frequency of 22 Htz some decades back. Both the frequencies are inaudible to human ears.

"This is giving us an insight into the culture of blue whales as they are clearly listening to each other's songs and changing them. It takes a conscious decision to make the calls deeper, so it is actually a reflection of what is going on in the population.

"These animals have a finite lung capacity, so their songs are a trade off between frequency and volume. They can either make the song really loud or really deep.

"As their numbers have slowly increased after the devastation caused by whaling, they are having to communicate over smaller distances so their songs don't need to be as loud and they can make them deeper," 'The Sunday Telegraph' quoted Prof Hildebrand as saying.

Prof Hildebrand and fellow researchers at California University have also discovered that the male species of blue whales in different parts of the world use different dialects in their songs.